Bonding of compressed graphite structures



United States Patent 3,475,244 BONDING OF COMPRESSED GRAPHITE STRUCTURES James A. Sanders, Jr., Houston, Tex., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Apr. 4, 1966, Ser. No. 539,632 Int. Cl. B32h 31/20, 7/12 US. Cl. 156-83 10 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for bonding compressed graphite structures and is more particularly related to a process for bonding relatively dense structures of compressed vermicular graphite to other relatively dense structures.

As disclosed in French Patent No. 1,395,964, most forms of natural and synthetic graphite may, upon proper treatment, be expanded by heat to form a light weight, particulate, vermicular form of graphite which may be compressed into more dense integral structures.

Compression of such expanded vermicular graphite along a single vector will produce a compacted integral structure having high electrical and thermal anisotropy. Both electrical and thermal resistivity are highest in the plane of compression and lowest in the plane perpendicular to that of compression. The anisotropy ratio between such planes increases with increasing compression up to or near the theoretical density of the graphite. Biaxial compression substantially reduces the anisotropy and if the vermicular graphite is compressed isostatically or triaxially little or no anisotropy can be produced. It can be seen, therefore, that structures having substantially any desired electrical and thermal orientation can e produced by compression of vermicular graphite in the proper manner.

To produce structures having acceptable conductive characteristics, it is usually desirable to apply a compression force of at least 30 p.s.i. To produce structures having an acceptable physical strength for most structural applications as well as good conductivity characteristics it is desirable to apply a compression force of 2000 psi. or more to achieve densities between 1.4 and 2.26 gm./cc. Due to the extreme mechanical difficulties involved in reducing a mass of very light weight vermicular graphite (usually having a density of about 0.005- 0.010 gm./cc.) to a relatively high density, relatively small structures are often made. It would be highly desirable in many instances to bond such structures together to produce larger structures or to produce structures having different electrical and thermal characteristics in different areas, or to bond thin graphite foils to other relatively high density materials such as sandblasted steel, rough surfaced ceramics, etc. Compacted graphite configurations, particularly that having a density above about 1.4 gm./cc., however, do not form a strong bond when compressed, even at high pressures, directly against another surface.

3,475,244 Patented Oct. 28, 1969 Now, unexpectedly, it has been found that by the process of this invention two or more graphite structures can be bonded together to form a unitary, monolithic structure. The present novel process likewise permits compressed graphite foil structures to be bonded to other relatively dense structures having roughened surfaces such as metals (e.g. iron, steel, copper, nickel, aluminum and the like), glass, wood, ceramics, plastics, cements and the like.

It is an object of this invention, therefore, to provide a process whereby relatively dense structures of compressed vermicular graphite may be bonded to other material by the application of pressure. A further object is to provide a process whereby two or more relatively dense structures of compressed vermicular graphite may be bonded to one another to form a monolithic structure. These and other objects and advantages of the present invention will become obvious from the following detailed description.

It has now been discovered in accordance with the present invention that a relatively dense structure of compressed vermicular graphite may be bonded to another relatively dense structure by treating the surface of the compressed graphite which is to be bonded with a re-expanding agent such as H HNO H O, acetone or the like, heating the treated surface sufficiently to produce an expansion of the surface to vermicular form and compressing the thus expanded surface of the graphite structure in contact with another surface to be bonded thereto.

In order to achieve the bonding of relatively high density compacts of compressed vermicular graphite to other such compacts or to other relatively dense structures by the present novel process, the surface of the compressed graphite to be bonded is first treated with a re-expandiug agent and heated to cause at least a portion of such surface to expand. By re-expanding agent is meant those agents which will enter the space between the laminate within the graphite to such an extent that upon heating, the surface of such graphite will expand. Suitable agents include intercalating agents, fuming nitric and sulfuric acids, mixtures of concentrated nitric and sulfuric acids, perhaloacids, bromine, and the like as well as liquids such as H O, acetone, and the like which are not true intercalating agents but which will be absorbed sufficiently by the graphite surface to produce expansion thereof upon heating. In some instances, it is also advantageous to roughen the surface of the compressed graphite mechanically or to heat the surface of the compressed graphite to a temperature of up to the boiling point of the expanding agent at the time of adding the expanding agent. Such heat treatment gives greater depth of penetration.

A temperature of 500 C. or more is usually required to produce adequate expansion and it is usually convenient to produce such temperature with a flame from methane, propane or acetylene and air or oxygen, electrical heat, etc.

Once the surface of the compressed graphite structure has been thus expanded, the structure may be tightly bonded to other relatively high density structures by compressing such expanded surface while in intimate contact with the surface of the structure or structures under a pressure of at least 30 p.s.i., preferably above a pressure of at least about 500 psi. and most preferably above about 1500 psi. Such pressure may be applied as a uniaxial force at right angles to the surfaces being joined or such pressure may be isostatic. If two relatively high density compressed graphite structures are to be joined, considerable advantage is gained by expanding both of the faces to be contacted prior to joining by compression.

3 4 The compressed graphite structures employed herein spraying such surface with fuming HNO and heating are prepared from vermicular graphite which may be with a propane flame until no acid odor was detectable. prepared, for example, by introducing an intercalating The surfaces to be joined expanded by such heating and agent such as fuming nitric acid, fuming H 80 or a were pressed together under a pressure of 17.000 p.s.i. mixture of concentrated nitric and sulfuric acids, HC103, 5 applied perpendicular to the bonding plane to form a etc. between the laminae of natural flake graphite and unitary, monolithic structure. expanding such graphite by the application of heat there- As a control, the remaining two blocks were likewise to. Usually, a temperature of 500 C. or more is recompressed together under 17,000 p.s.i. of pressure but quired to produce good expansion but under some cirno surface treatment and expansion was employed. cumstances, a lower temperature may be employed. Un- All of the so-joined samples thus prepared were broken der these conditions an expansion in volume of from 20 apart by applying a tensile load perpendicular to the bondto 400 times usually occurs to produce a very light ing plane. The result are shown in the following tabulation: weight, particulate vermicular form which is easily malle- Tensile able and compressible into shaped integral structures. I stress Wh1le the process of this lnvent on is useful to oin fi ggig fi f After g substantially any compressed vermlcular graphlte struc- Bond- Failure, ture to a relative dense structure, it finds particular ap- BlockA BlockB D plication in the joining of compressed vermicular graphite Control 1.47 1. 47 1. as 0 structures having a density above about 1.4 gm./cc. to re'expanded L41 o 1. 47 1. 44 1.9 6.86 other relatively dense structures and 1s espec1ally useful in the joining of compressed vermicular graphite com- In the same manner, sheets of compressed vermicular pacts to other compressed vermicular graphite compacts. graphite foil having a density of l.641.69 gm./cc. were The following examples are for the purpose of more bonded together under a pressure of 17,000 p.s.i Those fully illustrating the invention but are not to be confoils wherein the surface to be joined was first treated strued as limiting it thereto. and expanded produced a good bond but in the untreated EXAMPLE 1 foils no bonding was achieved. A series of ten uniaxially compressed blocks having EXAMPLE 3 densities ranging from 1.4 to 1.86 were prepared from A flat piece of sandblasted steel was clamped to a 0.5 vermicular graphite having a density of 0.005 gm./cc. inch square bar of compressed graphite under various Each block was about 2% inches square and had a thickclamping pressures and the voltage drop across the interness of 0.4 inch. Such blocks were bonded together in face between the steel and the graphite was recorded. Angroups by first heating the surfaces to be joined to a other 0.5 inch square bar of compressed graphite was first temperature of 60 C. and spraying the heated surface heated, treated with fuming NHO expanded with a prowith fuming HNO A propane flame was then contacted 5 pane flame as described in Example 1 and then clamped with the treated surface to produce the expansion thereto a fiat piece of sandblasted steel under various clampof. The surfaces of the blocks to be joined were placed ing pressures. Current was passed between the steel and in contact with one another and a pressure of 17,000 the graphite and the following results were obtained:

Millivolt drop/amp ratio at following clamp pressures Compressed Graphite Bar p.s.i. 80 p.s.i. 120 p.s.i. 160 p.s.i. 200 p.s.i.

Unexpanded (control) 50.0 21. O 24. 2 21. 9 21. 8 Expanded 36. 1 20. 0 14. 2 13.0 11. 8

p.s.i. was applied at right angles to suchsurfaces with I claim: sults a follows; 1. A process for the bonding of compressed vermicular graphite structures to other relatively dense structures Voltage which comprises gf (a) treating the surface of a compressed vermicular current graphite structure having a minimum density of f about 1.4 gm./cc. with a re-expanding agent, with (b) heating the treated surface to produce expansion fi gg thereof, Bonded Density over (0) directly contacting the resulting expanded graphite 5 2 2g3: Physical 3 surface of said compressed vermicular graphite struc- N0. gmJcc. Bonding gap ture with a second relatively dense structure having Block a roughened surface, and

1 2 1.41 Excellent 3.31mv. (d) compressing the thus contacted structures under Z 1: g? d a compression force of at least 30 p.s.i. to form an 1. 47 adhesive-free bond between said surfaces. {3% 2. The process of claim 1 wherein the treated graphite 1.80 surface is heated to at least 500 C. 52 3. The process of claim 1 wherein the compression 1.83 force is at least 2000' p.s.i.

4. The process of claim 1 wherein the compressed The denslty of the compressed structure after bonding vermicular graphite structure has a density of between was 1.91 gm./cc. To show the quality of the resulting about 1.4 gm./cc. and about 2.26 gm./cc.

joined structures the measured average voltage drop for 5. A process for bonding relatively dense compressed a 0.379 1n. section of such blocks in an area where there vermicular graphite structures to one another which was: no joint was 2.07 mv. comprises EXAMPLE 2 (a) treating the surface of at least one of each pair of compressed vermicular graphite structures to be 81x blocks were prepared by umaxrally compresslng joined with a re-expanding agent, vermicular graphlte having a density of 0.005 gm./cc. (b) heating each treated surface sufficiently to re-ex- Two pairs of such blocks were bonded together by heatpand such vermicular graphite,

ing the surfaces to be joined to a temperature of 60 C., (c) directly contacting the re-expanded graphite sur- 5 face of said compressed vermicular graphite structure with the graphite structure to be bonded thereto, and

(d) compressing the contacted structures under a compressive force sufiicient to produce a monolithic structure.

6. The process of claim 5 wherein all surfaces to be joined are treated with an expanding agent, heated to produce expansion and compressed together to form a monolithic structure.

7. The process of claim 5 wherein the treated graphite surfaces are heated to at least 500' C.

8. The process of claim 5 wherein the compression force is at least 500 psi.

9. The process of claim 5 wherein the compression force is at least 1500 p.s.i.

References Cited UNITED STATES PATENTS 6/1967 Olstowski 23-2O9.1 10/1968 Shane et al. 264109 ROBERT F. BURNETT, Primary Examiner J. D. FOSTER, Assistant Examiner US. Cl. X.R. 

